Pathogens and infectious disease

• Pathogens are disease-causing organisms.

• Main pathogen groups for this topic: viruses, bacteria, fungi, and protists.

• Archaea are not known to cause disease in humans.

• Infectious disease is caused by invasion and multiplication of pathogens in the body.

• Exam focus: distinguish pathogen from disease, and link each disease to a causative organism.

Primary defences: skin, mucous membranes, and clotting

• Skin acts as a physical barrier and a chemical barrier to pathogen entry.

• Mucous membranes protect internal surfaces exposed to the external environment.

• If skin is cut, blood clotting helps seal the wound and reduce pathogen entry.

• Platelets release clotting factors.

• A cascade pathway leads to conversion of prothrombin to thrombin and fibrinogen to fibrin.

• Fibrin forms a mesh that traps erythrocytes, producing a clot.

• Exam wording: clotting is a rapid protective response that seals cuts and helps prevent infection.

This diagram shows the intrinsic and extrinsic clotting pathways converging to form thrombin, which converts fibrinogen to fibrin. Use it to visualize the idea of a cascade leading to rapid clot formation after tissue damage. Source

Innate vs adaptive immunity

• The innate immune system responds to broad categories of pathogen.

• The innate immune system is non-specific and does not change during an organism’s life.

• The adaptive immune system responds specifically to particular pathogens.

• The adaptive immune system develops immunological memory.

• Because of memory, the adaptive response becomes faster and more effective after first exposure.

• For this topic, the key innate cell type to know is the phagocyte.

Phagocytes and the innate response

• Phagocytes move from the blood to sites of infection by amoeboid movement.

• They recognize pathogens, then engulf them by endocytosis.

• The engulfed pathogen is enclosed in a vesicle.

• Lysosomes fuse with the vesicle and release digestive enzymes.

• Pathogens are digested and destroyed.

• Exam sequence to memorize: recognition → engulfment → digestion.

The image shows a phagocyte recognizing a pathogen, forming a phagosome, and digesting it after lysosome fusion. It is ideal for memorizing the stepwise mechanism expected in short-answer questions. Source

Lymphocytes, antigens, and antibody production

• Lymphocytes are cells of the adaptive immune system.

• They circulate in the blood and are also found in lymph nodes.

• An individual has a very large number of B-lymphocytes, each able to produce one specific antibody.

• Antigens are recognition molecules that trigger antibody production.

• Most antigens are proteins or glycoproteins on the outer surface of pathogens.

• Antigens on erythrocytes can also trigger antibody production if blood of an incompatible blood group is transfused.

• Key idea: antigen specificity matches antibody specificity.

Activation of B cells and clonal selection

• B-lymphocytes and helper T-lymphocytes are both antigen-specific.

• A B cell is activated only when:

• it binds its specific antigen, and

• it also receives help from a helper T cell activated by the same antigen.

• Activated B cells divide by mitosis to form a clone.

• These clones differentiate into plasma cells.

• Plasma cells secrete large quantities of the same specific antibody.

• Exam link: this explains how a small number of specific B cells can produce a large antibody response.

Memory cells and immunity

• Immunity is the ability to eliminate an infectious disease from the body.

• It depends on the long-term survival of memory lymphocytes.

• Memory cells allow a more rapid and more effective response on later exposure to the same antigen.

• This is why secondary immune responses are stronger than first responses.

• Vaccination works by generating memory cells without causing the disease.

HIV and AIDS

• HIV is transmitted in body fluids.

• Know examples of transmission, such as sexual contact, blood-to-blood transfer, shared needles, and mother-to-child transmission.

• HIV infects and kills certain lymphocytes involved in immune defence.

• Reduction of these lymphocytes limits the body’s ability to produce antibodies and fight opportunistic infections.

• AIDS is the condition that results from severe immune system damage caused by HIV infection.

• Exam focus: explain that HIV does not kill all lymphocytes, but it reduces key lymphocyte populations enough to weaken immunity.

Antibiotics and antibiotic resistance

• Antibiotics are chemicals that block processes occurring in bacteria but not in eukaryotic cells.

• Therefore, antibiotics can treat bacterial infections without directly harming human cells.

• Antibiotics do not work against viruses because viruses lack the bacterial structures and metabolic pathways that antibiotics target.

• Resistance evolves in pathogenic bacteria when some bacteria survive antibiotic treatment and reproduce.

• Overuse and misuse of antibiotics increase selection for multiresistant strains.

• Careful antibiotic use is needed to slow the emergence of resistance.

• Exam trap: do not say bacteria “become resistant because they need to”; resistance spreads because natural selection favours resistant variants.

Zoonoses

• Zoonoses are infectious diseases that transfer from other species to humans.

• They are very common among human infectious diseases.

• Examples named in the syllabus: tuberculosis, rabies, Japanese encephalitis, and COVID-19.

• Be able to state that zoonoses can spread by different modes of transmission depending on the pathogen.

• Exam link: emergence of zoonoses shows the importance of interactions between human populations, animals, and pathogens.

Vaccines, immunization, and herd immunity

• Vaccines contain antigens, or DNA/RNA sequences coding for antigens.

• Vaccines stimulate immunity to a specific pathogen without causing the disease itself.

• This leads to formation of memory cells.

• Immunization is the process of becoming protected against a disease, usually by vaccination.

• Herd immunity occurs when a sufficiently high proportion of a population is immune.

• High population immunity greatly reduces transmission, helping protect non-immune individuals.

• Members of a population are therefore interdependent in disease prevention.

• Vaccination helps prevent epidemics by reducing the number of susceptible hosts.

Data skills: COVID-19 and disease trends

• Be able to evaluate data related to the COVID-19 pandemic.

• You may need to calculate percentage change.

• You may also need to calculate percentage difference.

• Always identify what the data show about transmission, cases, deaths, vaccination, or comparisons between groups.

• In data-based questions, distinguish trend, evidence, and conclusion.

Exam connections and common pitfalls

• Do not confuse innate immunity (non-specific, no memory) with adaptive immunity (specific, has memory).

• Do not confuse antigen with antibody.

• Do not say phagocytes produce antibodies; B cells/plasma cells do.

• Do not say antibiotics kill viruses.

• Do not define immunity as “not getting infected”; in IB Biology, immunity means the ability to eliminate the infectious disease from the body.

• In long responses, use the sequence: pathogen entry → primary defence → innate response → adaptive response → memory/immunity.